Electrophotographic marking is a well-known and commonly used method of copying or printing documents. Electrophotographic marking is performed by exposing a light image representation of a desired document onto a substantially uniformly charged photoreceptive member. In response to exposure by the light image representation, the photoreceptive member discharges so as to create an electrostatic latent image of the desired document on the surface of the photoreceptive member. A development material having toner is then deposited onto the electrostatic latent image so as to form a toner image. That toner image is then transferred from the photoreceptor onto a substrate, such as a sheet of paper. The transferred toner image is then fused to the substrate to form the completed document. Fusing of the toner image to the substrate is typically accomplished by a combination of heat and/or pressure. The surface of the photoreceptive member is then cleaned of residual developing material and recharged in preparation for the production of another document.
Multicolor images by electrophotographic marking can be produced by repeating the above described process once for each color of toner that is employed to form the composite toner image. For example, in one color process referred to herein as the REaD process (recharge expose and develop), a charged photoreceptive surface is exposed to a light image which represents the first component color of a composite color image. The composite color image is produced from a composite toner image of four layers of toner, the first typically being black, followed by magenta, yellow, and cyan. The resulting first electrostatic latent image is then developed with black toner particles to produce the black toner layer for the composite toner image. The charge, expose and develop process is then repeated to form a toner layer of the second component color of the composite color image. In an image-on-image process color (IOI), the subsequent component toner layers may overlay the previous component toner layers to thereby form a full spectrum of colors by their interaction. Alternately, the process can involve image-next-to-image (INI) wherein the component toner layers are positioned adjacent each other. Image-next-to-image is typically employed, for example, in highlight color printing. The INI process typically has two color toners wherein one component color is the highlight on the document. However, the INI process can clearly be implemented with additional color toners. In the IOI process, the color toner particles of the component toner layers are placed in a superimposed registration so that the desired composite color images are formed. The composite toner image in either the Read IOI process or REaD INI process is then transferred from the photoreceptive member and onto the substrate.
The REaD IOI and REaD INI processes can be implemented in a variety of configurations of an electrophotographic printing machine. In a single pass printing arrangement, the final composite toner image is produced in a single pass of the photoreceptive member through a printing machine having multiple charging development and exposure stations. Typically, four charging stations and four exposure stations are implemented to recharge, expose and develop each component color toner layer of the desired final four color composite toner image. Alternately in a multiple pass arrangement, the photoreceptive member cycles past a single charging station, a single exposure station, and multiple development stations. The photoreceptive member typically will cycle four times, one cycle for each component toner layer. In either configuration of an electrophotographic-printing machine, the composite toner image is subsequently transferred from the photoreceptive member to the substrate in a single step. The transfer can be directly to the substrate or via an intermediate toner support member such as a belt or drum.
The above described processes for single or multiple pass electrophotographic marking are not mutually exclusive but can be combined in various configurations. For example, a printing machine can have two charging stations and four developing stations. Two toner layers are added at each cycle of the photoreceptive member to result in a final four-color composite toner image in two cycles of the photoreceptive member.
Background print quality defects can be a serious problem with any color print architecture which requires recharge of the photoreceptive member. Recharge of the photoreceptive member occurs between development stages and also occurs during pre-transfer charging of the toner image on the photoreceptive member. In conventional electrophotographic systems, the non-image or non-developed image areas of the toner image typically have at least a small quantity of unwanted background toner developed onto them. Transfer of this background toner to the substrate results in the unwanted background print quality defects. The background toner is usually extensively of opposite polarity from the developed image area toner. This opposite polarity or wrong-sign toner can arise from aging toner, the charging processes or other actions that occur in the toner sump. In monochromatic electrophotographic systems, the wrong-sign background toner typically does not transfer effectively from the photoreceptive member to the substrate. The transfer charge treatment of the substrate in a monochromatic system is of a particular polarity that the substrate attracts the image area toner but repels the wrong-sign background toner developed in the background areas of the toner image.
The recharging of the photoreceptive member in for example the REaD IOI process between development stages, delivers the same charge polarity to the image and non-image or background areas. This recharging causes all the toner, both in the desirable image areas and in the background areas, to take on the same charge polarity. The charging of all the toner on the photoreceptive member with the same polarity, regardless of location also occurs during pre-transfer charging to prepare the toner image for transfer to the substrate. In either the recharge step or pre-transfer charge step, the wrong-sign background toner is recharged to the same polarity as that of the image toner resulting in transfer of the background toner along with the image toner to the substrate. This background toner transfer degrades print quality.
Toner splatter is an additional print quality defect that can occur from the REaD/IOI process. Toner splatter occurs when highly charged toners are developed on top of multiple layers of previously developed toners. Fringe fields are generated between the top most toner and the photoreceptor that pull or drive the toner particles in the top most toner layer from their preferred location. The toner particles in the top most toner layer move from the top toner layer along the electric field and onto the background areas surrounding the image area. The result of the movement of the toner particles of the top most toner layer is a bleeding or halo of the color of the top most toner layer onto the photoreceptive member surrounding the preferred composite toner image.